The use of inkjet printing in offices and homes has grown dramatically in recent years, which can be attributed in part to drastic reductions in the cost of inkjet printers and the substantial improvements in print resolution and overall print quality. While the print quality has drastically improved, research and development efforts continue toward improving image quality and permanence of inkjet images because these properties fall short of those produced by other printing and photographic techniques. Inkjet inks used in inkjet printing are typically aqueous-based and are formulated by dissolving a colorant, such as a dye or pigment, in an aqueous ink vehicle. The ink vehicle comprises additional components depending on the application and desired properties of the color inkjet ink.
Plain paper was initially used as a print medium in inkjet printing. While plain paper was adequate for printing text, it did not provide high resolution images. One recent development in inkjet technology has been the ability to print images that have a similar image quality to that of a photograph, which is commonly referred to as photographic image quality. A photographic quality image has saturated colors, high gloss and gloss uniformity, freedom of grain and coalescence, and a high degree of permanence. To provide the photographic quality image, the inkjet ink and print medium must be compatible with one another. In addition, the inkjet ink should have low puddling, good crusting resistance, good stability, low dye bleed, and a rapid dry time. The print medium used to print the photographic quality image must be fast drying and resist smearing, air, light, and moisture. In addition, the print medium should provide good color fidelity and high image resolution.
However, current print media commonly do not provide all of these properties. These print media generally include an ink receiving layer on a substrate, such as a paperbase or a photobase layer. The ink receiving layer includes multiple coatings that are formed from inorganic or organic materials, such as inorganic particles or organic polymers. The print media are categorized into two groups: porous media and swellable media. Porous media have an ink receiving layer that is formed from porous, inorganic particles bound with a polymer binder. Inkjet ink is absorbed into the pores of the inorganic particles and the colorant is fixed by mordants incorporated in the ink receiving layer or by the surface of the inorganic oxides. Porous media have a short drytime and good resistance to smearing because the inkjet ink is easily absorbed into the pores of the ink receiving layer. However, porous media do not exhibit good resistance to fade. In swellable media, the ink receiving layer is a continuous layer of a swellable, polymer matrix. When the inkjet ink is applied, the inkjet ink is absorbed by swelling of the polymer matrix and the colorant is immobilized inside the continuous layer. Since the colorant is protected from the outside environment, swellable media have greater resistance to light and dark/air fade than the porous media. However, the swellable media generally have reduced smearfastness and a longer drytime than porous media.
Some of the biggest failures of inkjet printed images are dye bleed (migration), hue shifts, and changes in optical density (“OD”). These undesirable properties are caused by migration of the dyes on the print medium and result in printed images having an unacceptable image quality. Dye bleed is the migration of the inkjet ink from its original location on the print medium. Dye bleed occurs under many conditions but is particularly prevalent at high temperature and high humidity, which is referred to herein as humid bleed. Printed images that are resistant to humid bleed are said to be humid fast. Hue shifts are caused by the different migration of cyan, magenta, and yellow dyes on the print medium. Hue shifts can occur in a matter of days and are easily observed by a user, which leads to user dissatisfaction with the inkjet printing products that he or she is using.
To reduce dye migration on plain paper, dyes having a boronic acid group have been used. The boronic acid group reacts with a cis-diol group to form a covalent bond. Plain paper includes cis-diol groups and, therefore, the boronic acid group forms a covalent bond with the plain paper. The covalent bond fixes the dye to the plain paper so that the dye does not migrate.
Dye migration has also been reduced by using dyes with low water solubility in the inkjet inks. Since these dyes are not very water soluble, the dyes precipitate when they are applied to the print medium, resulting in reduced dye migration. However, the amount of precipitation depends on the type of print medium that is used and, therefore, this solution is not optimized for use with all dyes. Furthermore, using dyes with low water solubility undesirably affects pen reliability because the dye has a tendency to precipitate and clog the pen. Another proposed solution has been to add mordants to the print medium. However, each mordant is typically specific for only a few dyes and, therefore, is not optimized for use with all dyes. In addition, the mordant causes problems with the image quality.
It would be desirable to provide an inkjet ink that provides reduced dye migration when printed on a coated print medium. The inkjet ink should also produce printed images having improved dye bleed, hue shifts, and OD changes when exposed to water or a humid environment.